Donor-acceptor Association Equilibrium
Charge-transfer complexes are formed by weak association of molecules or molecular subgroups, one acting as an electron donor and another as an electron acceptor. The association does not constitute a strong covalent bond and is subject to significant temperature, concentration, and host, e.g., solvent, dependencies.
The charge-transfer association occurs in a chemical equilibrium with the independent donor (D) and acceptor (A) molecules:
Quantum mechanically, this is described as a resonance between the non-bonded state |D, A> and the dative state |D+...A->. The formation of the oxidative state is an electronic transition giving rise to the colorful absorption bands.
The intensity of charge-transfer bands in the absorbance spectrum is strongly dependent upon the degree (equilibrium constant) of this association reaction. Methods have been developed to determine the equilibrium constant for these complexes in solution by measuring the intensity of absorption bands as a function of the concentration of donor and acceptor components in solution. The methods were first described for the association of iodine dissolved in aromatic hydrocarbons. The procedure is called the Benesi-Hildebrand method, named after the authors of the study.
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